The overall goal of this section is to better understand the effects of the host cell and other co-factors on human retroviral replication. In particular, the monocyte/macrophage, a reservoir of HIV-1 infection in AIDS patients, was shown to be able to restrict HIV-1 expression. In THP-1, a monocytoid cell line, two classes of restricted HIV-1 expression were seen: 1) low-level and 2) no expression (latent). Latently infected cells made infectious virus after 5-azacytidine exposure. In contrast, LPS and several cytokines could increase viral production in restricted cells but could not activate latently infected cells. Thus, both restricted and latent states of HIV expression probably occur in monocytes by different mechanisms. In restricted HIV- 1 expressing THP-1, the absence of a DNA binding protein complex in the HIV-1 enhancer was seen concomitant with the production of markedly less viral RNA. This absence of binding was localized to the NF-kappaB region of the HIV enhancer with the 65+50 kD NF-kappaB heterodimer being preferentially lost. Addition of purified NF-kappaB protein to nuclear extracts from cells with restricted expression restores this binding. Also, treatment of these nuclear extracts with sodium deoxycholate restored heterodimer binding suggesting that a specific inhibitor of NF- ~B was present in these cells. Thus, the binding of 65+50 kD heterodimer to the HIV-1 enhancer can be negatively regulated in monocytes. Studies of latently infected THP-1 showed that these cells could be induced to express infectious virus by coculture with Con A- activated T cells, isolated from normal, HIV negative donors. Freshly isolated adherent monocytes from asymptomatic seropositive individuals were examined and found to contain latent HIV. After coculture of these monocytes with Con A-activated T cells from HIV negative normal donors, these latent monocyte cultures expressed virus. HIV produced by activated latent monocytes infects T cells and can lead to viral-induced pathology. These results suggest a role for infected monocytes producing little or no virus in viral persistence and spread particularly in infected individuals who remain asymptomatic for years before progressing to overt AIDS.